Add overrides and switch to nullptr keyword

Changes generated with clang-modernize.

Additionally, fixed some struct-vs-class usage to make clang happy.

Change-Id: Ic6ef2427401ff1e794d26f21f7b44868fc75fb72
diff --git a/libs/hwui/font/CacheTexture.h b/libs/hwui/font/CacheTexture.h
index a107c7a..5d3f959 100644
--- a/libs/hwui/font/CacheTexture.h
+++ b/libs/hwui/font/CacheTexture.h
@@ -54,7 +54,7 @@
     CacheBlock* mPrev;
 
     CacheBlock(uint16_t x, uint16_t y, uint16_t width, uint16_t height):
-            mX(x), mY(y), mWidth(width), mHeight(height), mNext(NULL), mPrev(NULL) {
+            mX(x), mY(y), mWidth(width), mHeight(height), mNext(nullptr), mPrev(nullptr) {
     }
 
     static CacheBlock* insertBlock(CacheBlock* head, CacheBlock* newBlock);
@@ -146,7 +146,7 @@
     }
 
     uint16_t* indices() const {
-        return (uint16_t*) 0;
+        return (uint16_t*) nullptr;
     }
 
     void resetMesh() {